1. Field of the Invention
The present invention relates to a control device and a control program product for an engine used in a motorcycle or an automobile, particularly the present invention is suitable for applying to an engine having a valve driving mechanism in which a cam having its cam profile axially varying continuously is slid along the axis of the cam shaft so as to control continuously a valve lift characteristic to be steplessly variable.
2. Description of the Related Art
As a valve driving mechanism provided to an engine, there discloses in Japanese Patent Application Laid-open No. 4-187807, for example, an art of a valve driving mechanism in which a cam having its cam profile axially varying continuously is slid along the axis of the cam shaft so as to control continuously a lift amount and lift timing of an intake valve or an exhaust valve to be steplessly variable.
When such a cam is applied to the intake valve especially, by continuously varying a lift characteristic of the intake valve to be steplessly variable, an intake air amount can be controlled, so that an intake resistance can be reduced, removing the throttle valve of an intake path. As a result, an engine output can be increased.
By setting the cam profile so as to the intake valve will shut early in a low load range of engine, an air-fuel mixture is expanded adiabatically after the intake valve is shut, and further, compressed adiabatically. Owing to this expansion, an intake temperature falls, and the intake temperature just before the ignition also falls to be lower than the case that the valve is shut late. Thereby, a knocking is prevented, at the same time, an expansion ratio can be maintained high, so that the heat efficiency can be improved by a miller cycle engine in which the expansion ratio is higher than a compressed ratio.
If the lift amount itself is reduced, a mechanical loss can also be reduced, as a result, the good fuel economy can be obtained.
In this type of valve driving mechanism, the lift amount is determined according to an opening-degree of accelerator and an engine speed so as to control the sliding of a cam. When the engine runs in an idling state, namely, in a state the accelerator is shut down completely, the intake air amount fluctuates due to some conditions, there exists the fears that an engine rotation is revved up fast and adversely stalled.
When the feedback control of a cam position is performed only for controlling the air amount, the delay for moving the cam position incurs a hunting of engine rotation.
In this type of valve driving mechanism, the increasing condition of engine temperature is lower than the condition of an engine having the commonly used two-dimensional cam, therefore, a temperature regulation is important for preventing deterioration of exhaust gas, or for improving the engine output.
If an intake pipe simply leaving out the generally-used throttle valve etc. which controls through the whole range of engine rotation is provided to the engine, the air-fuel mixture especially in the small intake amount may not be sufficiently obtained.